GRAPTOLITES OF NEW YORK, PART 1 669 



Description. Primary disk subcircular, relatively large (diameter 5 mm 

 where the rhabdosorae has a length of 6.5 mm), nema short, filiform, very 

 thiu. Sicula inconspicuous, short (1.3 mm) and relatively broad. Branches 

 horizontal, angle of divergence 180°; attaining great length (incomplete 

 specimens measure upward of 40 cm), increasing very gi'adually in width 

 from .7 mm to 2.4 mm; slightly flexuous. Thecae 9 in 10mm; short and 

 broad (proportion of length to 

 width, 2:1 in the proximal por- 

 tion and 3:1 in the distal portion), 



attaining a length of 2.2 mm in Fig. eg Didymograptus extensus Hall sp. Proxlmal 



^ part of rhabdosome. Reverse view. Deep kill. x5.25 



the distal part ; in contact for one 



half of their length in the proximal and two thirds in the distal parts; 

 inclined at an angle of 35° to 40". Outei* and apertural margins straight, the 

 latter normal to the axis of the theca (inclined at 130" to the axis of branch). 

 Position and localities. At the Deep kill this species has l)een found 

 only in graptolite bed 2 (Upper Tetragraptus zone), Avhere it, in association 

 witli Tetragraptus similis, covers one layer and in others is very 

 common and associated with D i c h o g r a p t u s o c t o ]:> r a c h i a t u s, T e t r a- 

 g r a p t u s f r u t i c o s u s and Phyllogi-aptus ilicifolius. Hall 



obtained his types from the Quebec group 

 at Point Levis (Tetragraptus zone) ; Lap- 

 worth records it also from the zone with 



Fig. 6.3 Didymograptus extensus Hall T3 i i i i • j_- 



sij. Fragment of proximal part of branch. Deep C 11 y i 1 O g T a p t U S a U U a , Citing aS 

 kill. X5.2.5 



locality " 3 miles above St Anne " ; and 

 Ami lists D. extensus from several localities in the neighl^orhood of 

 Levis. In Great Britain it has been recognized in the Middle Skiddaw 

 slates of the Lake district ; in the St David's district and the Lleyii 

 peninsula of Wales (Hick's Lower Arenig) ; in Shropshire ; and in the 

 Ballantrae district in Scotland. Brogger records it from the Phyllograptus 

 shale of the Christiania region ; Tornquist found it in the zone of 

 Isograptus gibberulus (lowest zone of his Phyllo-Tetragraptus 

 beds) of Scania ; and Freeh mentions it also from the Hunneberg in 



